How do I evenly emit particles from a surface?
Increasing the subdivisions of the mesh should help, you will just need to make sure the subsurf modifier is above the particle system and then check 'Use Modifier Stack' in the particles panel. 

Another option is to add some random velocity to the particles in the Velocity panel. It seems (based on brief testing) that adding random velocity will produce nicer results when emitting from volume as opposed to faces.